Lighting applications account for about 30% of the electrical energy consumption in the United States. With increasing interest in energy conservation, lighting systems that use less energy and are easy and cost-effective to install are becoming more important. One effective way to reduce energy consumption of lighting is to use dimmable fluorescent lighting systems. Newer lighting systems can control light output and energy consumption by adjustment of lighting levels throughout the day, reducing energy usage when light is not needed. Existing dimmable fluorescent lighting systems require extra, low-voltage control wiring to provide control signals to the ballasts contained within lighting luminaires. The present invention allows the lighting control signals to be sent over the same wiring that is used to provide AC power to the ballasts. By using the existing wiring, the changeover from conventional lighting is greatly simplified and installation costs are significantly reduced.
Presently there are two types of dimmable ballasts for fluorescent lighting. The first type utilizes a phase-controlled thyristor dimming control to reduce the AC current provided to a special lighting ballast. These ballasts respond by creating suitable internal dimming signals that are then used to vary light intensity. This type of ballast can use conventional, two-wire AC power wiring, allowing them to be installed easily. However, the abrupt modulation of the AC line current causes severe distortion, greatly reducing the power factor, and increasing the harmonic content of the line current. This type of ballast, therefore, can introduce significant problems into the power distribution when a significant number of the ballasts are installed within a large building.
The second type of dimming ballast uses separate low-voltage control wiring to provide dimming signals to the ballast. The control voltages are typically DC signals that may vary from 0 volts (fully dimmed) to 10 volts (full brightness). The AC power is handled on separate conventional AC power wiring. Because the control function is separated from the power line, no distortion is introduced by the control system to the AC power, making this type of ballast suitable for use in large installations. The drawback to their use is the added complexity of installation, since the additional low-voltage control wiring is needed.